The present invention relates to a method for fabricating a thin film semiconductor device in which a thin film transistor having an active layer of a semiconductor thin film formed on an insulating substrate is integrally formed and more particularly to a laser annealing technology for implemented a aiming at crystallization of the semiconductor thin film after forming it on the insulating substrate.
As a part of a method for lowering the processing temperature of manufacturing steps of a thin film semiconductor device, a crystallization annealing technology using a laser beam is being developed. It enables crystallize a semiconductor thin film in a cooling step after applying an energy beam (laser beam) to locally heat the non-monocrystalline semiconductor thin film such as amorphous silicon or polycrystalline silicon formed on a transparent substrate. A thin film transistor is integrally formed utilizing the crystallized semiconductor thin film as an active layer (channel region) thereof.
A thin film semiconductor device is suited to a driving substrate and the like of an active matrix type display panel and is being actively developed in recent years. Then, while it has been strongly requested to increase the size and the lower the cost of the transparent substrate in applying it to the display panel, the above-mentioned crystallization annealing using the laser beam is drawing attention as a method that meets this request. Because the semiconductor thin film may be crystallized at relatively low temperature by applying laser beam, a relatively low cost transparent substrate such as low fusion point glass may be adopted. In this case, a technology of preliminarily heating the insulating substrate by using a heater or the like to assist the crystallization annealing using the laser beam has been proposed and is described in Extended Abstracts of the 1991 International Conference on Solid State Devices and Materials, Yokohama, 1991, pp. 623-625, for example.
Generally, it is necessary to raise the temperature to 600.degree. C. or more to crystallize amorphous silicon. Accordingly, if the substrate is heated preliminarily to raise the substrate temperature up to around 400.degree. C. beforehand, energy density of the laser beam may be saved that much and it works favorably on the crystallinity and uniformity of the semiconductor thin film. The semiconductor thin film having a large grain size and an excellent crystallinity may be obtained by carrying out the crystallization annealing together with the preliminary heating of the substrate. It has been known that the thin film transistor formed by this film is highly efficient as it has a high carrier mobility and an excellent gate voltage swinging characteristic. However, there has been a problem concerning the throughput in the preliminary heating method using the heater because it took a considerable time to raise the temperature of the transparent substrate to a predetermined temperature. For example, it took several minutes to ten-odd minutes of preliminary heating time to raise the temperature of the transparent substrate made from normal glass or the like to 400.degree. C.
Instead of the substrate preliminary heating (furnace annealing) using the heater (electric furnace), a so-called lamp annealing for preliminarily heating the substrate by applying illuminant light from a lamp in a batch has been also proposed. While a halogen lamp containing a large amount of heat rays such as infrared rays is used in the lamp annealing in general, it is inefficient and is not practical because the semiconductor thin film formed on the transparent substrate such as glass barely absorbs infrared rays and its temperature does not rise, though it can effectively heat a silicon wafer used in fabricating normal ICs.
Meanwhile, the laser beam which is generally linear along the scan direction is applied pulse-wide intermittently while partially overlapping each other in the above-mentioned laser irradiation step. The semiconductor thin film may be crystallized relatively uniformly by overlapping the laser beams.
In applying the laser beams by partially overlapping each other in the laser annealing, a desirable distribution of sectional intensity of energy of the laser beam is as flat as possible. However in reality, the intensity is weak at the peripheral portion of the laser beam as compared to that at the middle portion. If the laser beams are applied by overlapping each other in such a state, regions of non-uniformly crystallition emerge like strips in the region between the shots of the laser beams and where the edge of the laser beam is applied. A crystal grain size is generally small in this strip part of non-uniformly crystallition. Therefore, if a thin film transistor is formed by using the strip part as its channel region, it turns out to be a low performance transistor because of the small crystal grain size. Accordingly, when a plurality of thin film transistors are integrally formed on an insulating substrate, operating characteristics vary among each individual thin film transistor because the non-uniformly crystallized strip part exists. Further, there has been a problem in the reliability of the thin film transistor because its mobility is low and it deteriorates readily. If the insulating substrate on which such thin film transistor is integrally formed is used as a driving substrate of an active matrix type display unit for example, it would cause a problem in the uniformity of the image quality.